1. Field of the Invention
The present invention relates to an information storage medium and a method of manufacturing the same, in which information is recorded and erased by applying a light beam, such as a laser beam, to a recording layer so that the irradiated portion of the recording layer undergoes phase transformation.
2. Description of the Prior Art
Optical disks of a phase-transformation type are conventionally known as an erasable optical disk. In these optical disks, information is recorded and erased by utilizing reversible phase transformation of an irradiated portion of a recording layer between crystalline and amorphous structures, for example, which should be caused when a laser beam is applied to the recording layer.
Materials capable of such phase transformation include, for example, semiconductors, semiconductor compounds, and intermetallic compounds, such as Te, Ge, TeGe, InSe, SbSe, SbTe, etc. These materials alternatively assume two phases, crystalline and amorphous, depending on temperature. The complex index of refraction, which is given by N=n-ik, varies depending on the phase. Accordingly, information is recorded and erased by reversibly changing the phase through heat treatment with use of a laser beam (S.R. Ovshinsky: Metallrgical Transactions 2 641 1971).
Besides the method described above, there is a system in which information is recorded and erased by reversible phase transformation between different crystal structures through irradiation with a laser beam (Japanese Patent Disclosure No. 61-134944). InSb is a well-known example of material capable of such phase transformation.
A thin film of an InSb alloy develops into a fine crystal structure when it is exposed to a low-power laser beam with a relatively long pulse width. If a high-power laser beam with a short pulse width is applied to the alloy film, on the other hand, the film develops into a relatively coarse crystal structure. These two crystal structures have different complex indexes of refraction, and their states are discriminated by a difference in reflected light quantity, for example, obtained when information is read by applying a laser beam.
According to the aforementioned method in which information is recorded and erased by phase transformation between crystalline and noncrystalline structures, the stability of the noncrystalline structure at the recording portion (indicated by recording mark hereinafter) may be low. In the recording and erasing method based on the phase transformation between different crystal structures, however, the stability of the recording mark is satisfactory. If a recording layer formed of an InSb film is left under the atmosphere of a temperature of 70.degree. C. and a relative humidity of 90% for about three months after recording, the recording mark remains stable. In this respect, the latter method is superior to the former.
Meanwhile, the optical disks of the phase-transformation type are generally constructed as shown in FIG. 1, without regard to the aspect of phase transformation. Protective layer 2, formed of a dielectric material stable both chemically and thermally, is formed on substrate 1 of glass or an organic resin which is transparent to light. Recording layer 3 is formed on protective layer 2, and protective layer 4 of the same material as layer 2 is formed on layer 3. For protection against flawing, moreover, protective layer 5 of an ultraviolet-setting resin is formed on layer 4.
Among these layers, protective layers 2 and 4 have the following functions.
(1) When a laser beam is applied to recording layer 3, layers 2 and 4 prevent layer 3 from abrasion by heat, and from being deformed by repeated recording and erasing operations.
(2) In reading operation, layers 2 and 4 enhance signals by utilizing optical interference.
(3) In the case that the phase transformation occurs between crystalline and amorphous structures, layers 2 and 4 facilitate radiation of heat from recording layer 3 and hence, uncrystallization by rapid cooling, during the laser-beam irradiation.
(4) In the case that the phase transformation occurs between different crystal structures, layers 2 and 4 thermally insulate recording layer 3, thereby restraining heat radiation therefrom, and help layer 3 to be gradually cooled to solidify after melting.
In general, SiO.sub.2 is used for these protective layers.
However, the optical disks of the crystal-to-crystal phase-transformation type, especially those optical disks whose recording layer is formed of an InSb alloy, have the following problem. Suppose protective layers 2 and 4 are formed of SiO.sub.2, and a amorphous film, at an as-deposited state, is intended for initial crystallization by continuous laser-beam irradiation. In this case, if the angular velocity of the optical disk is as low as 100 to 150 rpm, initialization is accomplished by applying a laser beam of 6-mW output or thereabout to one and the same track portion once or twice. If the angular velocity ranges from 900 to 1,000 rpm, the same track portion must be irradiated as frequently as six to eight times, even though the output of the laser beam is as high as 10 mW.
Also in the erasing operation, information on the recording portion can be erased by applying a low-output laser beam of 6 mW or thereabout only once, if the angular velocity of the optical disk is 300 rpm or less. If the angular velocity is 1,000 rpm or more, however, part of the recorded information remains unerased seen though a laser beam of 10 mW is applied to the same recording portion twice or thrice.
Such a problem may be solved by high power output of the laser beam. Currently, however, commercially available miniature semiconductor lasers can deliver an output of only 30 mW at the most. Even if the actual output is 30 mW, an output of only about 10 mW, which is substantially equivalent to the aforesaid output, can be applied as an effective output to the disk surface through an optical system.